Among the array of jobs in public service, district attorney is one of the most relevant and impactful.

A district attorney and his staff typically help citizens at their most vulnerable, after they or a family member have been victimized by crime. How those people are treated goes a long way toward restoring their self-confidence, as well as their faith in government and the justice system.

Perhaps the most coveted such job in Massachusetts is in Middlesex County, the most populous in the state, covering a swath of the northeast that ­includes such diverse communities as Cambridge and ­Lowell.

Scott Harshbarger, Thomas F. Reilly, and Martha Coakley all went from from Middlesex district attorney to attorney general, before Harshbarger and Reilly ran for governor, and Coakley took a shot at the US Senate.

All that explains why it was a shock to the political system Thursday night when the current Middlesex district attorney, Gerard T. Leone Jr., 50, ­announced to his staff that he would not seek reelection in 2014.

A string of potential successors immediately started gauging their support, including Middlesex Sheriff Peter ­Koutoujian; Middlesex County Clerk of Courts Michael ­Sullivan; Assistant Attorney General John Verner; Senator Eileen Donoghue of Lowell; Representative David Linsky of Natick; Marian Ryan, a 34-year Middlexes prosecutor; and former prosecutor Kerry Ahern of Lowell.

After two terms in office, with no challenger the last time he ran and none on the horizon, Leone decided to walk away from public service on his own terms. In his career, the Hopkinton Democrat personally prosecuted some of the most memorable cases in the state, including those of attempted shoe bomber ­Richard Reid and nanny Louise Woodward.

Breaking the news to his staff at an awards and recognition ceremony for them told the public something about Leone’s humble personality, as well as his commitment to the people who have worked for him.

“Like my predecessors, I, too, will not run for reelection to a third term as Middlesex district attorney in 2014,” ­Leone said in his speech. “However, unlike my predecessor Middlesex district attorneys, I am not running for another elective office, and, in fact, I intend to leave government service when I leave this office. I do not now know what I will do when I leave. But, I do know that anyone who is interested in competing to succeed me needs time to compete. I want that field of potential successors to be as rich and deep as the job deserves. And, therefore, I am expressing my intentions publicly now.”

He went on to say that “this job was the only elected office that I wanted” and that he ­“endured” the elective process to hold it.

Then, in a personal aside, he closed his remarks to the staff by quoting Albert Einstein.

“As someone who is much smarter than I am once said, ‘I must be willing to give up what I am, in order to become what I will be,’ ” Leone said. “Other than a husband and father, I am not yet sure what that will be when I am no longer a district attorney.”

Donoghue said Leone’s ­effectiveness has been rooted in his proactiveness.

Ahern, a Lowell criminal ­defense attorney who previously worked for Reilly, Coakley, and Leone, said Leone has an epic work ethic, too.

“I used to get texts from him at 5:30 in the morning; that’s when he starts his day,” said ­Ahern.

In mid-September, the Globe contacted Leone amid chatter that he might be under consideration for athletic director at Pennsylvania State University as the school tried to rebuild its image after the Jerry Sandusky child sex abuse case.

Leone said he had not been contacted about the post, but he also did not shy away from discussing why he would be an appropriate choice for it.

There were signs at that time that Leone was nearing the end of his tenure in political office. Chief among them was that his fund-raising had dropped off.

A report covering the period through Aug. 31 showed he had $86,000 cash on hand. At the same point during his last reelection cycle, he had $191,000. Yet Leone explained that he still had more than $500,000 when adding in interest-bearing CDs and a money market account.

“I’d rather spend my time working the job I enjoy than raising money,” he said during that interview. “At the same time, I have supporters and commitments that would put me well over $1 million if I needed the money.”

That said, Leone also suggested another campaign was no sure bet.

“I’m not going to be district attorney for life,” he said, “so, at some point, I have to think about doing other things.”

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.

Thanks & Welcome to Globe.com

You now have unlimited access for the next two weeks.

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.